Optical isolators are used in a variety of applications in optical communication systems. Generally, optical isolators are used to prevent reflective portions of transmitted signals from re-entering the transmitting device. Many older prior art designs prevent reflections from re-entering a transmitting device in a polarization-selective manner. However, in certain circumstances where a transmission system causes uncontrollable changes in polarization, the polarization state of a signal may be unknown, and thus, this earlier polarization dependent designs are not considered to be practical. Thus, as of late the trend has been to provide optical isolators that are polarization independent.
One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 issued Jul. 23, 1991 in the name of Jameson and entitled Polarization Independent Optical Isolator. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, an optical signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator which provides 22.5° of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber. In practice, Jameson's single stage isolator described above, may provide adequate isolation; however, in some instances, increased isolation may be required. Such additional isolation has been known to be provided by using a multi-stage optical isolating device, however known prior art multi-stage devices tend to be bulky and costly to manufacture, often requiring nearly double the number of optical components that a single stage device requires.
As with most electronic and optical devices today, there is an increasing focus on miniaturizing and as well on reducing the cost of manufacturing devices. At times, these objectives are mutually compatible, such that when a device is miniaturized, its cost of manufacture decreases.
In an effort to overcome many known disadvantages of prior art optical isolators U.S. Pat. No. 5,768,005 discloses a relatively compact multi-stage optical isolator wherein a retro-reflector 62 in FIG. 1 of this application, is used to return a beam incident thereon in a backwards direction through the isolating elements to an output port of the optical isolator.
Although the optical isolator described performs its intended function of providing multiple stages of isolation, the structure of the device does not lend itself to a practical way of inexpensive manufacture or assembly. For example due to the presence of the corner cube prism, making a single, bulk, large isolating block and “dicing” it into a plurality of uniform complete optical isolators is not practicable.
Prior art US patent application US2003/0058536 discloses a single stage optical isolator having a right angle prism at an end thereof for folding the beam propagating therethrough, however the isolator does not provide multiple stages of isolation. Furthermore the input ports lie along a line that is offset from a line that the output ports lie along. This is not particularly useful for coupling to planar waveguide light wave circuits (PLCs) where the waveguides lie along a common line.
It has been found to be highly desirable by the inventors of this application to have an isolator that provides at least two stages of isolation, which is compact, and which can be diced into thinner dual stage isolators and wherein the input and output ports lie along a single line. By providing the input and output ports along a single line, such a multi-port two stage isolator can be used at the edge of a PLC to couple to a linear array of waveguides. By so doing, two-stage optical isolation can be “essentially” in-line with and abutting a waveguide chip wherein many ports can achieve optical isolation dependent upon matching the spacing of the ports on the dual stage optical isolator and the waveguides on the chip.
With this in mind, and in view of the known disadvantages and limitations of prior art devices, it is an object of this invention to provide a reflective multi-stage optical isolator that lends itself to large scale manufacturing with little active alignment of components and which has a plurality of input ports and a plurality of output ports aligned along a same line. In a preferred embodiment a two-stage optical isolator is described which overcomes the limitations of the prior art.
This invention has several very significant advantages over the prior art dual stage isolator disclosed in U.S. Pat. No. 5,768,005. Manufacturing a single thick isolator and actively aligning a large right-angle Porro prism at an end thereof, allows subsequent dicing of the large thick isolator into n thin isolators with no further active alignment of the isolating components. This large scale manufacturability saves build-time, minimizes active alignments required; allows sheets of components to be polished and glued together in a single step, rather than having to actively align each isolator.
Therefore, it is an object of this invention to provide a multi-port dual stage optical isolator having ports that lie along a single line that is less costly to manufacture by way of requiring less expensive components and by way of providing a structure that requires many fewer assembly stages in the manufacture of plural isolators.
It is an object to provide a large multi-stage multi-port isolating element which can be manufactured by polishing and bonding or gluing plural components together and then cutting or “dicing” the large element into separate multi-port multistage optical isolators.
It is an object of this invention to provide a method and structure whereby a single active alignment allows a plurality of optical isolators to be aligned, without having to actively align respective components of each isolator.
It is a further object of the invention to provide an embodiment that provides many two-stage optical isolators each having a plurality of ports wherein each isolator is polarization independent and which requires fewer than n active alignments of optical components for n isolators.